1. Field of the Invention
The present invention relates to a display device and an electronic device.
2. Description of the Related Art
In recent years, display devices using liquid crystal panels or organic EL panels have been under active development. These display devices are broadly classified into display devices in which only transistors for pixel control (pixel transistors) are formed over a substrate and a scanning circuit (driver circuit) is included in a peripheral IC and display devices in which a scanning circuit is formed over the same substrate as pixel transistors.
A display device in which a driver circuit is integrated with pixel transistors is effective in reducing the frame width of the display device or the cost of the peripheral IC. However, transistors used in the driver circuit are required to have better electrical characteristics (e.g., field-effect mobility (μFE) or threshold voltage) than the pixel transistors.
A silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor. As another material, an oxide semiconductor material has been attracting attention (for example, Patent Documents 1 and 2). For example, attention has been drawn to a transistor including a semiconductor thin film formed using an amorphous oxide that contains indium (In), gallium (Ga), and zinc (Zn) and has an electron carrier concentration of less than 1018/cm3.
A transistor using an oxide semiconductor for a semiconductor layer has higher field-effect mobility than a transistor using for a semiconductor layer amorphous silicon which is a silicon-based semiconductor material. Hence, the transistor using an oxide semiconductor operates at high speed and is suitably used for the display device in which a driver circuit is integrated with pixel transistors. Besides, manufacturing steps of the transistor using an oxide semiconductor are easier than those of a transistor using polycrystalline silicon for a semiconductor layer.
However, the transistor using an oxide semiconductor for a semiconductor layer is disadvantageous in that entry of impurities such as hydrogen or moisture into the oxide semiconductor generates carriers to change the threshold voltage of the transistor, which is one of the electrical characteristics of the transistor.